Where can I find scripts to play with?

Stack Apps is a Q&A site for apps, scripts, and development with the Stack Exchange API. People discuss about developing around SE, and publish their projects there. You can find a lot of userscripts in the scripts section.

One of my scripts isn't working

beeshyams asked me this question before, but I wasn't a Firefox user then, so I installed Firefox fresh and installed that userscript, and found nothing wrong.

When you encounter some issues with userscripts, try removing and reinstalling them, as well as clearing/resetting your Firefox addon, and see if the issue resolves by itself. Usually this solves 99% of them.

Userscripts are a big help for those who use mobiles mostly to browse SE sites. Scripts such as Auto review are very useful in reviewing and commenting with customized canned comments. Scripts from Stack Apps can further enhance ease of mobile reviews
– beeshyamsDec 16 '18 at 7:24